1. Field of the Invention
The present invention relates to a novel method for preparing 1-alkyl-3-fluoroalkyl-1H-pyrazole-4-carbonyl chlorides, a useful precursor for the preparation of fungicides, by means of reductive dehalogenation, starting from N-alkyl-3-haloalkyl-5-halopyrazolecarbaldehyde.
2. Description of Related Art
1-Alkyl-3-haloalkylpyrazolecarbonyl chlorides are important building blocks for preparing plant protection active ingredients, particularly SDHI fungicides. Unsubstituted pyrazolecarbonyl chlorides are typically prepared by reacting carboxylic acids with a chlorinating agent. One advantage of this method is that the corresponding carboxylic acids are readily accessible and are thus available on an industrial scale. This prerequisite is not satisfied for the preparation of substituted pyrazolecarbonyl chlorides, since the corresponding carboxylic acids are not readily accessible. 1-Alkyl-3-haloalkylpyrazolecarbonyl chlorides are typically prepared in a multi-stage transformation starting from fluoroalkylacetoacetate:

Steps 1 and 2 are particularly technically demanding. In step 1 a very large excess (2 to 3 equivalents) of acetic anhydride is required in order to ensure a complete transformation. The isolation of the product from step 1 and separation thereof from the excess acetic anhydride or acetic acid is costly and very time-consuming. In step 2, the formation of the second isomer (up to 15%) and the separation thereof renders the method difficult and costly.